Copper Choice and Copper Efficiency for High-Frequency PCB Design

Expert woodworkers spend immeasurable amounts of time making sure that finished wood has an exceptionally smooth surface before staining and varnishing can occur. Although my dreams to become the popular host of a DIY woodworking show have diminished to simply attempting to produce a high-quality solid wood bookcase for my wife, I have retained my obsession with achieving a smooth surface.

Sanding rough wood is a chore, to say the least. Whether using a hand or electric sander, the sandpaper gleefully skips over each rough spot. The sander requires more normal force to move the sandpaper because the bumps and grooves resist momentum. Each variance in the wood grain angle requires additional changes in sanding pressure.

As we design any printed circuit board and develop circuits that operate at higher frequencies, achieving a very smooth copper surface becomes one of several important factors. When frequency increases, the effective resistance of the copper traces increases. The impact of the effective resistance becomes greater with a rough copper surface.

High-Frequency Currents and Copper Surface Roughness

The interaction of high frequencies with copper traces adds an interesting twist that includes several factors. The problems begin as every upslope and downslope of the rough copper foil lengthens the distance that current flows and increases the propagation path. As frequency increases, the resulting increase in the resistance of the copper combines with the longer distance to introduce loss. Thus, the signal integrity of the PCB suffers.

The problems continue with thermal conductivity (k) —or the capability of a material to conduct heat. Materials that have low thermal conductivity have a low rate of heat transfer measured in watts per meter per degree Celsius (W/M -oC). High thermal conductivity translates to higher heat transfer. As an example, copper has a higher thermal conductivity of 386 W/M-oC and carries more heat away at a higher rate. In contrast, FR-4 laminate serves as a thermal insulator and has a much lower thermal conductivity value. With all this, the thermal conductivity of copper that has a rough surface becomes a factor. When a circuit has greater loss, it generates more heat.

Copper surface roughness also affects the dielectric constant of the copper. Because the dielectric constant (dk) measures the effect of a material on capacitance, any changes in surface area impact capacitance and the characteristic impedance of the material. Increased roughness results in an increase of surface area and an increase in the effective dielectric constant and an increase in capacitance.

Proper copper planning begins in the schematic with an understanding of your design

With the increase in capacitance, the characteristic impedance decreases. In terms of design, the change in characteristic impedance affects matched impedance circuits. Materials with a lower dielectric constant require an increase in conductor width to maintain the characteristic impedance and decrease the opportunity for loss.

The dielectric constant also is the ratio of the permittivity of the dielectric to the permittivity of a vacuum—a constant medium. Permittivity describes the effect of the copper on an electric field. In addition, permittivity discloses the ability of a material to polarize in response to an applied field. An increase in polarization by the material in an applied field of an established strength causes the dielectric constant to increase. However, copper roughness changes the propagation constant and especially affects the permittivity of thinner laminates.

Skin effect drives current to the surface and loss increases. In turn, the resistance of a transmission line increases as frequencies increase. Another phenomenon called the skin depth effect distributes alternating current within a conductor so that the largest amount of current density accumulates near the surface. Redistributing current in the signal and return paths causes the signal and return currents to move closer together.

Heavy Copper Effects Toward Dielectric Constants

Skin depth measures the penetration of a signal into a trace. Approximately two-thirds of the signal energy penetrates to one skin depth. At low frequencies of around 10MHz, signal energy penetrates to .0008 inches. Increasing the signal to 10GHz reduces one skin depth to .000028 inches.

If a conductor has a skin depth thinner than the typical RMS thickness of the copper or two microns, most current flows in the rough part. At this point, the inverse relationship between skin depth and frequency becomes interesting. Skin depth decreases inversely with the square root of frequency. As the frequency increases, the alternating current takes the path that presents the least impedance and goes to the surface.

This inverse relationship produces resistance that increases with the square root of frequency. For frequencies less than 1GHz, minimal dielectric loss occurs. At frequencies above 1GHz, skin depth decreases while the series resistance and conductor loss increase. When frequencies move to the 10GHz range, current only flows along the extreme outer surface of the conductor and the remainder of the trace becomes unused. With current flowing only at the outer surface, skin effect creates a resistive drop that affects performance.

The impact on skin depth causes current density to decrease exponentially in areas below the surface. Changing the AC current in the conductor allows a changing magnetic field to develop. Producing the changing magnetic field induces a changing electric field that suppresses currents in the center of the conductor.

Redistributing the current also reduces loop inductance and spreads the current within each conductor. Moving the signal and return currents closer together increases the mutual inductance between the two currents. Spreading the current in each conductor reduces self-inductance.

Smooth Copper Choices for High-Frequency Design Management

Along with trace width and length, a smooth copper surface changes the relationship between current density and frequency. While the copper has some roughness due to the need for adhesion, circuit board designers need to make the correct copper foil choice for high-frequency circuits. Available with coarse, moderate, and fine grain structures, Electro Deposited (ED) copper has a vertical grain structure or rougher surface that becomes much more susceptible to losses caused by the skin effect. However, ED copper has a greater bond strength.

Rolled Annealed (RA) provides a smoother, denser surface that works best for rigid-flex PCBs. When compared to ED copper, RA copper has a horizontal grain structure has a much lower risk for high-frequency insertion losses caused by skin effect and has better thermal characteristics. In contrast to the rougher ED copper, however, the smooth RA copper has lower adhesion properties.

Making sure you have the output files to get your devices produced is invaluable.

The choice between different types of copper depends on the application and project costs. While high-frequency rigid-flex designs may use RA copper, the cost of RA copper may become prohibitive for other projects. Design teams use PCB layout software to select the appropriate copper type, build the mechanical and electrical connections for the printed circuit board, and create the structure for a working design. As the PCB layered structure grows, it consists of multiple conductive and insulating layers making PCB stackup and PCB stackup design invaluable.

Tools within the circuit board layout software optimize the placement of parts and copper and precisely define critical board elements. Maximizing copper can occur through manual processes that place copper shapes as objects or automatic copper pours that define a boundary and connect everything inside the boundary. With the manual method, the layout software assigns the objects to a net and uses continuity checks to find any shorts or errors. The automatic methods connects objects—such as vias, traces, pads, and nets--within the same net according to design rules.

PCB Designers Keeping Up with Layout Advancements for Manufacturing

From a consumer perspective, rapid advancements in electronics technologies seem almost expected and commonplace. Devices integrate numerous functions and run at faster signal speeds. For example, the high-frequency anti-collision warnings that have nearly become a standard in new automobiles no longer seem new. For PCB designers, however, the near-constant introduction of new products and capabilities presents numerous challenges. Those challenges include the need for enhanced mechanical strength at connectors and vias, the need for withstanding thermal strains, and the needed for increased current carrying capacity.

One of the solutions involves the use of heavy copper for power applications. The use of heavy copper builds resistance to thermal strains, increases mechanical strength, and increases the current carrying capacity of the PCB. Fabricators can use heavy copper and multiple copper weights to reduce the overall product footprint while maintaining on-board heatsinks to transfer heat away from the circuit.

While the manufacture of heavy copper circuits occurs through the same etching and plating techniques used for standard copper PCBs. However, the heavy copper circuits require specialized etching and plating methods. Other fabrication methods use a hybrid approach that combines heavy and standard copper to increase circuit density and integrate high current circuits and control circuits. Those methods require greater attention to seamless connections through precisely controlled tolerances.

To help you meet these challenges, Altium Designer allows a design team to consider material parameters, relative permittivity, dielectric thickness, trace width, and loss tangent. These tools, coupled with other valuable resources, give you the ability to consider high-performance PCB materials with a consistent response to high signal speeds, different product applications, and fabrication processes. As an example, Altium Designer calculates the correct trace width by find the stable operating temperature for the circuit. With this information, Altium Designer determines the maximum current that a trace can carry and the best type of dielectric material for the application. In addition, Altium Designer defines the distance between traces and pad sizes that align with increases in plate thicknesses.

While working with Altium Designer to generate your manufacturing output materials, consider its Active BOM to also organize your components and materials as well. To learn more about how Altium can help you optimize your PCB design, talk to an Altium expert today.